Carbonyl sulfide (OCS) has recently emerged as a tracer for terrestrial
carbon uptake. While physiological studies relating OCS fluxes to leaf
stomatal dynamics have been established at leaf and branch scales and
incorporated into global carbon cycle models, the quantity of data from
ecosystem-scale field studies remains limited. In this study, we employ
established theoretical relationships to infer ecosystem-scale plant OCS
uptake from mixing ratio measurements. OCS fluxes showed a pronounced diurnal
cycle, with maximum uptake at midday. OCS uptake was found to scale with
independent measurements of CO2 fluxes over a 60 m tall old-growth
forest in the Pacific Northwest of the US (45∘49′13.76′′ N,
121∘57′06.88′′ W) at daily and
monthly timescales under mid–high light conditions across the growing season
in 2015. OCS fluxes were strongly influenced by the fraction of downwelling
diffuse light. Finally, we examine the effect of sequential heat waves on
fluxes of OCS, CO2, and H2O. Our results bolster previous
evidence that ecosystem OCS uptake is strongly related to stomatal dynamics,
and measuring this gas improves constraints on estimating photosynthetic
rates at the ecosystem scale.

Carbonyl sulfide (OCS) is the most abundant sulfur gas in the atmosphere,
with a mean atmospheric concentration of ∼ 500 ppt (parts per
trillion), and therefore a significant part of the tropospheric and
stratospheric sulfur cycles, with implications for the global radiation
budget and ozone depletion (Johnson et al.,1993;
Notholt et al., 2003). The dominant sink of atmospheric OCS is vegetation
(Kesselmeier and Merk, 1993; Kettle et al., 2002; Montzka et al., 2007, and
references therein), through rapid and irreversible hydrolysis by the
ubiquitous enzyme carbonic anhydrase (Protoschill-Krebs et al., 1996; Protoschill-Krebs and Kesselmeier, 1992).
Recent advances in spectroscopic technology have enabled continuous in situ
measurements of OCS on timescales that are relevant to understanding
stomatal function at the leaf-scale
(Stimler et al., 2010a, b), branch scale (Berkelhammer et
al., 2014), and the ecosystem scale
(Kooijmans et al., 2017; Wehr et al., 2017). An important distinction between OCS and
CO2 cycling is the absence of a retro-flux from actively
photosynthesizing leaves (OCS emissions have been reported from stressed
crops following severe fungal infection;
Bloem et al., 2012). However, the normalized leaf uptake ratio of OCS :CO2 (LRU;
Sandoval-Soto et al., 2005)
is relatively constant at medium to high light levels
(Maseyk et al., 2014; Stimler et al., 2010), making it an excellent proxy for
quantifying plant productivity (gross primary productivity – GPP; Asaf et al., 2013; Billesbach et al., 2014; Blonquist et al., 2011). On the other
hand, both uptake and emissions of OCS from soils have been identified
(Whelan et al., 2016; Sun et al., 2015; Maseyk et al., 2014; Kesselmier et
al., 1999). While ecosystem-scale measurements of OCS continue to establish
links between OCS uptake and GPP in different ecosystems (for a
comprehensive list of ecosystem-scale studies, readers are referred to Fig. 2
in Whelan et al., 2018), inconsistencies persist. For
example, in an oak–savanna woodland in southern France, Belviso et al. (2016) found that OCS exchange was strongly influenced by photosynthesis
during early morning hours, while meaningful values of LRU could only by
calculated for a few days in the early afternoons.
Commane et al. (2015) were
unable to explain midsummer emissions of OCS in a midlatitude deciduous
forest. Uncertainties highlighted above argue for field-scale measurements
of OCS in a variety of ecosystems, particularly as OCS flux predictions have
recently been incorporated to inform estimates of plant productivity in
global carbon cycle models
(Campbell et al., 2017a; Hilton et al., 2017; Launois et al., 2015).

OCS fluxes have not been previously reported for old-growth forests,
although a recent study using flask samples inferred a large uptake of OCS in
coastal redwood forests in northern California
(Campbell et al., 2017b). Rastogi et al. (2018) found large
drawdowns in mixing ratios of OCS at an old-growth forest in the Pacific
Northwest of the US and significant uptake of this gas by various components
of the ecosystem (leaves, soils, and epiphytes). In this study, we report
estimates of OCS fluxes from an old-growth forest and place them in the
context of ecosystem carbon and water cycling. Additionally, we investigate
the response of CO2, H2O, and OCS fluxes to changes in the fraction
of downwelling diffuse radiation as well as heat wave events through the
growing season. Technological constraints posed limitations on measuring
fast-response OCS fluxes, so instead we combine continuous in situ
measurements of OCS mixing ratios above and within the canopy with
established theoretical equations for OCS uptake
(see Berry et al., 2013; Commane et al., 2015; Seibt et al., 2010) to
characterize OCS fluxes using a simple empirical model and compare them with
ecosystem uptake of CO2 from colocated eddy covariance measurements.

2.1 Site description

Measurements were made at the Wind River Experimental Forest (WR), located
within the Gifford Pinchot National Forest in southwest Washington state,
USA (45∘ 49′13.76′′ N, 121∘57′06.88′′ W; 371 m a.s.l.). The site is well
studied and described in great detail
(Paw U et al., 2004; Shaw et al., 2004; Wharton and Falk, 2016; Winner et al.,
2004). The climate is classified as temperate oceanic with a strong summer
drought. The forest is 478 ha of preserved old-growth evergreen needleleaf
forest, with dominant tree species of Douglas fir (Pseudotsuga menziesii) and western hemlock
(Tsuga heterophylla). The tallest Douglas fir trees are between 50 and 60 m, while the
shade-tolerant hemlocks are typically between 20 and 30 m high. Maximum rooting
depth is 1–2 m for the tallest, dominant Douglas fir trees although most of
the root biomass is concentrated in the first 0.5 m (Shaw et al., 2014). The
cumulative leaf area index (LAI) is estimated to be 8–9 m2 m−2 (Parker et al., 2004).
Additionally, the ecosystem hosts a large diversity of mosses, lichens and
other epiphytic plants, which play an important role in canopy OCS dynamics
Rastogi et al., 2018). The soils are
volcanic in origin, although most of the forest surface is comprised of
decaying organic matter (Shaw et al., 2004).

2.2 Study period

Measurements reported here are from between 18 April and 31 December 2015. However,
in early November an intake line at the top of the canopy was
damaged after a rainstorm. Measurements continued at the other intake
heights (see Sect. 2.4 and 2.9). Therefore, ecosystem fluxes and related
analyses in this study cover 136 days between 18 April and 31 October while
chamber-based soil fluxes are reported for the months of August–December.
Gaps in the time series due to analyzer maintenance correspond to
26–28 June, 6–17 July, 4–7 August, 24 August, and 4–7 October. April–October roughly
corresponds to most of the growing season, although at this site GPP usually
peaks early in March–April, when soil moisture is high and ecosystem
respiration flux is low, while plant productivity is typically severely
light and temperature limited in the months of November–December (Wharton
and Falk, 2016). Environmental conditions during the measurement campaign
are shown in Fig. 1 and represent a typical Mediterranean-type climate,
with temperature peaking in July and minimal to no measured rainfall between
June and September. This results in high summertime atmospheric vapor
pressure deficit (VPDa), and soil moisture declines steadily through the
summer period, with some recharge following rare precipitation events in
September and then more commonly in October. The measurement period also
encompasses three distinct heat waves, characterized by anomalously high air
temperatures and midday VPDa values (often exceeding 4 kPa). We examine the
response of OCS and CO2 fluxes during these heat waves.

Figure 1Environmental conditions at Wind River during the measurement
campaign: daily mean air temperature (a), precipitation (b) midday VPDa (c), and
soil moisture measured at three depths (d) are shown.

2.3 CO2 and H2O eddy flux measurements

Carbon, water, and energy fluxes have been collected since 1998 at the Wind River AmeriFlux tower
(US-wrc; Paw U et al., 2004). For further details, readers are referred to
Falk et al. (2008; instrumentation and data processing) and
Wharton et al. (2012) and Wharton and Falk (2016) for multiyear carbon and water
flux measurements and synthesis.

2.4 OCS measurements

A commercially available off-axis integrated cavity output spectroscopy
analyzer manufactured by Los Gatos Research Inc. (LGR; model 914-0028) was
deployed at the base of the tower in an insulated and temperature-controlled
shed. The instrument measures mixing ratios of OCS, CO2,
H2O, and CO simultaneously at a maximal scan rate of 5 Hz. The
system uses a 4.87 µm cascade laser coupled to a high-finesse
800 cm3 optical cavity, and light transmitted through the cavity is
focused into a cooled and amplified HgCdTe detector. OCS is detected at
∼ 2050.40 cm−1, CO2 at 2050.56 cm−1, CO at ∼ 2050.86 cm−1, and H2O at ∼ 2050.66 cm−1.
Pressure broadening associated with changes in the concentration of water
vapor in the samples is corrected for in the analysis routine. Air was
sampled through 0.25 in. diameter PFA (polyfluoroacetate) tubing using a diaphragm pump at a flow
rate of 2 L min−1, from inlets located at 70 (at the height of the
eddy flux instrumentation), 60 (canopy top), 20, 10, and 1 m. The
sampling frequency was 0.1 Hz, and the sampling interval was 5 min. The
first minute of each sampling interval was removed to avoid any
inter-sample mixing. The remaining data were checked for temperature and
pressure fluctuations inside the measurement chamber, and a moving window
filter was used to eliminate any sudden outliers in the data. Mixing ratios
were aggregated to provide hourly means. For detailed information regarding
instrumentation and the measurement, readers are referred to Rastogi et
al. (2018), Berkelhammer et al. (2014), and Belviso et al. (2016).

2.5 Calibration

Calibration was performed using ambient air stored in
insulated tanks as a secondary reference. Air was sampled into the analyzer
daily, and tank pressure was routinely monitored to check for leaks. Glass
flasks were randomly sampled from calibration tanks and measured against an NOAA GMD (Global Monitoring Division ) reference standard. Cross-referencing revealed that the accuracy of
the measurement was within the reported minimum uncertainty of the
instrument (of 12.6 pmol mol−1; Berkelhammer
et al., 2016).

2.6 Thermal camera measurements

Leaf temperatures were measured from
28 October 2014 to 28 January 2016 using a FLIR A325sc thermal camera
(FLIR System Inc., Wilsonville, OR), in which a FLIR IR 30 mm lens (focal
length: 30.38 mm; field of view: 15∘× 11.25∘)
was installed. The thermal camera has a pixel resolution of 320 × 240.
Within the field of view (FOV), spot sizes of a single pixel are 0.83 cm from 10 m distance and 8.3 cm from 100 m distance. Manufacturer-reported
errors in original measured thermal temperatures are ±2∘C
or ±2 % of the measurements. The camera model is identical to one
used in another study at an AmeriFlux site in central Oregon (US Me-2), and
the detailed specifications can be found
in Kim et al. (2016). To monitor a larger canopy region, a pan–tilt unit (PTU) was used
for motion control, allowing multiple canopy thermal image acquisition
within one motion cycle. We used a FLIR PTU-D100E (FLIR System Inc.,
Wilsonville, OR; (http://www.flir.com/mcs, last access: 21 November 2018) to move the thermal
camera vertically and horizontally at specific pan and tilt angles. We
selected five pan–tilt angle (PT) positions representing the upper canopy
(i.e., ∼ 40 to 60 m above the forest floor) to estimate leaf
temperatures in this study.

2.7 Diffuse light measurement and analyses

An SPN1 Sunshine Pyranometer
(Delta-T Devices Ltd., Cambridge, UK) was installed at the top of the
canopy and collected direct and diffuse shortwave downwelling radiation from
April to December 2015. Measurements were made every 1 min and then
aggregated to hourly means. We limited our analyses of diffuse radiation
data to include only midday hours (between 11:00 and 13:00) to minimize the
influence of solar angles on diffuse radiation fractions. We defined three
distinct periods based on the ratio of diffuse radiation to total incoming
solar radiation (henceforth referred to as fdiff). Data were characterized as clear if
fdiff<0.2, as partly cloudy if fdiff>0.2 and fdiff<0.8, and as overcast if
fdiff>0.8.

2.8 OCS flux estimation

where FOCS, FH20, ΔOCS, and ΔH2O are the
fluxes and gradients of OCS and H2O, respectively, and SOCS is the
change in storage flux of OCS. A change in storage flux is subject to large
uncertainties, and estimates have been shown to vary depending on the
averaging time and vertical resolution of the storage profile
(Yang et al., 2007), horizontal resolution, and site heterogeneity
(de Araújo et al., 2010; Nicolini et al., 2018) as well as canopy decoupling
(Jocher et al., 2018). Since large parts of the canopy at the site are decoupled from the bulk air
at all times (Pyles et al., 2004), we inferred change in storage as the height-integrated change in the time
derivative of mixing ratios between the canopy top and above the canopy.
Following Seibt et al. (2010) and Berry et al. (2013), we assume that OCS
is irreversibly and rapidly consumed inside leaves, such that the gradient
between ambient air and the leaf interior effectively reduces to the ambient
measured OCS mixing ratio:

(2)ΔOCS=χOCSa-χOCSl=χOCSa,

where ΔOCS is defined as the gradient of OCS between ambient air
and the leaf intercellular spaces (χ is the mixing ratio of OCS, and
superscripts a and l refer to ambient and leaf, respectively). In our study,
χOCSa is the measured mixing ratio at the canopy top (60 m)
instead of above canopy (70 m) to account for turbulent transport between the
canopy top and air that is above the canopy top. We use the vapor pressure
deficit (VPD) as the corresponding gradient for H2O, under the key
assumption that the intercellular leaf surfaces are saturated with water
vapor. While VPD is usually calculated using air temperature, a more
accurate calculation can be performed with leaf temperatures, which can
deviate significantly from air temperatures (Kim et al., 2016), leading to
significant differences between the VPD of ambient air and that at the leaf
surface (Figs. 2a and 3d in this study). Previously, leaf temperatures have
been inferred from sensible heat fluxes, wind speed, and air temperatures
(e.g., Wehr et al., 2017); here, we use explicit measurements of leaf skin
temperatures to estimate leaf–air VPD (VPDl). Analogous to Eq. (3),

(3)ΔH2O=χH2Ol-χH2Oa=ei-eaP=VPDlP,

where ei is saturation vapor pressure in the leaf sub-stomatal cavity
(kPa), using leaf skin temperature, ea is the actual vapor pressure (kPa),
P is the measured atmospheric pressure (kPa) at the tower top, and χH2Ol and χH2Oa
(ppth) are the leaf and ambient H2O mixing ratios at the canopy top. Finally, since gradients of OCS
and H2O are estimated between ambient air and the leaf intercellular
spaces, these are normalized by the ratio of diffusivities of these two
species in air (Seibt et al., 2010; Wohlfahrt et al., 2012).

FH20 was measured using eddy covariance at the tower top (70 m). In high-LAI forests with minimal exposed soil, such as those of the Pacific
Northwest, fluxes of FH20 can be treated as a good proxy for
transpiration, since soil evaporation is minimal. We excluded rainy days, as
well as two days following rainfall, to only capture periods when FH20
can be assumed to be dominated by transpiration. Equation (1) was evaluated
only under the condition FH20>0.2 mmol m−2 s−1. We
restricted our analyses to daytime, when OCS flux is assumed to be related
to leaf CO2 uptake (Maseyk et al., 2014; Wehr et al., 2017).

Leaf relative uptake was calculated following Seibt et al. (2010).

(4)LRU=FOCSGPPχCO2χOCS,

where GPP was estimated from CO2 fluxes measured at the tower top,
using a nighttime based partitioning approach (Reichstein et al., 2005),
which was optimized for the site (Falk et al., 2008). Finally, canopy conductance
(Gc) was estimated using a simple flux-gradient approach with the assumption
that the canopy (or ecosystem) acts as a single big leaf.

(5)Gc=FH2OVPDlP

2.9 Surface fluxes

A long-term automatic soil survey chamber (Li-Cor 8100-104, 20 cm diameter) was installed at three 0.03 m2 surface sites
in series, within 1 m of each other. All plastic and rubber parts had
been removed from the chamber and replaced with materials compatible with
OCS measurements: stainless steel, PFA plastic, and Volara foam. Blank
measurements were performed in the laboratory before deployment, and OCS
concentrations in the chamber were found to be statistically
indistinguishable from incoming ambient concentrations. The stainless-steel
chamber top opened and closed automatically on a timer. Gas was drawn
through the chamber via a pump downstream of the analyzer, and the 3 L min−1
flow rate was confirmed with a mass flow meter. When the chamber
was open, ambient near-surface air was observed. When the chamber was
closed, trace gas concentrations reached a stable state for at least 2 min during the 10 min incubation time. The difference between the
ambient concentration and the stable closed-chamber concentration were used
to calculate the surface fluxes of OCS and CO2.

(6)Fforest floor=McΔχA-1,

where Mc is the measured flow rate into the chamber (converted from
L min−1 to mol−1 using the ideal gas law), ΔX is the difference
between mixing ratios of OCS or CO2 in ambient air and the chamber, and
A is the surface area of the chamber. The minimum flux detectable with this
method was 1.2 pmol m−2 s−1 uptake or production.

Care was taken to select sites characteristic of the surface, which was
generally springy and covered in a mat of mosses and lichen. Surface flux
observations were made at site 1 from 6 to 16 July, site 2 from 13 August to
7 October, and site 3 from 6 November to 2 December 2015. The first site
was visually similar to the subsequent two sites at the surface, though the
chamber base of the first site was installed into the moss layer and a
barely decomposed fallen tree. When a soil sample was attempted to be
extracted from the footprint of the chamber base, several liters of intact
wood litter were removed. The influence of the developed soil on site 1 is
therefore considered minimal. Site 2 was selected nearby and observations
were made until a dominant tree fell on the soil chamber. The chamber was
repaired and re-installed a month later at site 3, and observations continued
without incident until the chamber was removed in advance of the soil
freezing.

3.1 Ecosystem fluxes

The composite diurnal cycles for CO2, water
vapor, and OCS and fluxes are shown (Fig. 2a–d). The total ecosystem flux of
OCS (FOCS; Fig. 2d) follows a pronounced diurnal cycle that peaks
at midday. The vertical profile of mixing ratios measured throughout
the canopy is also shown (Fig. 2b). OCS mixing ratios are highest at the
canopy top and lowest near the forest floor, but mixing ratios increase from
the early morning to midafternoon. Together these processes are indicative
of ecosystem uptake and downward entrainment of boundary layer air (Rastogi
et al., 2018a). The shape of the FOCS curve is very similar to those of
net and gross carbon fluxes (Fig. 2b–c), although FOCS was consistently
negative during daylight hours. Leaf relative uptake, a ratio of
FOCS: GPP normalized by the mean mixing ratios of OCS :CO2, showed a
strong light dependence (Fig. 2e). High-light, midday values ranged between
3–4, which is higher than those observed in other forest systems (Kooijmans
et al., 2017; Wehr et al., 2017) but well within the spread of values
obtained in a recent meta-analyses of OCS studies for vegetated ecosystems
(Whelan et al., 2018). The diurnal cycle was found to be asymmetric, with
peak values observed in the early morning, when stomatal conductance is
likely to be high (Winner et al., 2004), but GPP is limited by low light. It is important to note that
LRU is likely influenced by large amounts of epiphyte and understory
vegetation, which assimilate OCS even at times when ecosystem CO2
uptake is low or zero. Epiphytic assimilation of OCS is highly influenced by
moisture content (Gimeno et al., 2017) and is typically higher through the night and in the early
mornings at this site (Rastogi et al., 2018). Moreover, in tall old-growth
forests, leaf area is vertically distributed over a much larger part of the
canopy compared to other forests (Parker et al., 2004). While leaves at the
canopy top exercise tight stomatal control to limit water loss and minimize
hydraulic failure (Woodruff et al., 2007), leaves lower down in the canopy, including those of understory
vegetation, likely impose less stomatal control of transpiration
(Winner et al., 2004). Lower-canopy leaves may therefore continue to disproportionately assimilate
OCS, even under low rates of carbon assimilation (as CO2 uptake is
additionally light limited).

3.2 Daily and seasonal dynamics

Daytime fluxes of OCS (estimated as fluxes
when PAR (photosynthetically active radiation) was higher than 100 µmol m−2 s−1) were correlated
to independent estimates of GPP (Fig. 3a), and the uptake of both OCS and
CO2 reduced as soil moisture declined. Variability in the relationship
between fluxes of OCS and CO2 and soil moisture was related to VPD,
which fluctuated as a response to changing cloud cover (discussed later in
Sect. 3.4).

Figure 3FOCS was linearly correlated to GPP (plotted as a negative
quantity to show ecosystem uptake a), while both FOCS and GPP reduced as a
function of decreasing soil moisture (b–c). Data presented here are midday
means; data in (b–c) are colored according to VPD.

Ecosystem uptake of OCS and CO2 (as well as GPP) was highest in April
(Fig. 4a) and declined as the soil drought progressed (Fig. 4f). The mean
monthly maximum OCS flux was estimated as -61±6 pmol m−2 s−1, while daily mean maximum GPP over this period was
estimated as 10±1µmol m−2 s−1 (plotted as a negative
quantity in Fig. 4b to show ecosystem uptake). While the steepest declines
in FOCS, NEE (net ecosystem exchange of CO2) and GPP happened between the months of May and June,
FOCS continued to decline through the rest of the summer, with a
minimum in August, and remained low in September and October. CO2
fluxes flattened between June and September, before declining again in October.
While the uptake of OCS and CO2 followed similar patterns, the H2O flux
remained high until midsummer (Fig. 4c) and decreased in August, presumably
due to a combination of high VPD (Fig. 4d) and declining soil moisture (Fig. 4f),
as plants exercised greater control over stomata. This can be clearly
seen in the seasonal cycle of canopy conductance (Gc; Fig. 4e). Mean monthly
Gc was highest in the months of April and May and then declined in response
to increasing VPD and decreasing soil moisture, before increasing again
slightly in September and October following soil recharge and decreased VPD
due to precipitation events. In October, soil water recharge, several
rain-free days (Fig. 1), and lower VPD (Fig. 4d) do not result in increased
gas exchange, likely due to the downregulation of photosynthesis (Eastman and Camm, 1995),
induced by photo-protective changes in the xanthophyll cycle (Adams
and Demmig-Adams, 1994).

3.3 Surface fluxes

Forest floor OCS fluxes were observed from three sites in
series and within 1 m of each other. Site 1 had approximately twice the OCS
uptake compared to the subsequent two sites and had a substantial layer of
intact woody debris under the chamber footprint. Sites 2 and 3 had OCS fluxes
similar to previous surface fluxes reported for forests (Whelan et al.,
2018). For all sites, there was no clear diurnal pattern. For site 2, uptake
immediately following chamber installation was higher (∼ 6 pmol m−2s−1)
than fluxes later on (all <6 pmol m−2 s−1) when temperatures were lower (Fig. 5). Site 3 did not have
high uptake after chamber installation and had consistent fluxes between
the detection limit and −6.2 pmol m−2 s−1 for the first few
weeks. When ambient air temperatures dropped below freezing, uptake remained
unchanged, except for the largest uptake observed (6 to 12 pmol m−2 s−1)
during two events when average air temperature fluctuated
from a cooling to warming trend. Soil temperature never dropped below
freezing during the experiment and was generally colder over time. We did
not observe any OCS emissions from the chamber-based measurements,
consistent with recent studies that find that cooler, moist
(Maseyk et al., 2014; Sun et al., 2016; Whelan et al., 2016), and radiation-limited
(Kitz et al., 2017) soils do not emit OCS.

Surface CO2 emissions exhibited a relationship with temperature, where
highest production (∼ 25 µmol m−2 s−1) corresponded with temperatures ∼ 15 ∘C and maximum
flux values decreased for warmer and colder temperatures. CO2 emissions
had a diurnal pattern, with lowest emissions at night and maximum emissions
in late morning to mid- afternoon. No obvious relationship emerges from
CO2 emission and OCS uptake, though the high OCS uptake events in late
November and early December have a linear relationship with CO2
emissions. For sites 2 and 3, the ratio of OCS emission to CO2
production, normalized by the concentration of OCS and CO2 in the
closed chamber, was between −0.25 and −3.5 with a mean of −1. In contrast,
the same ratio for site 1 varied from −5 to −19 with a mean of −10.

Figure 6Midday VPDl, FOCS, NEE, and GPP plotted against the
fraction of diffuse downwelling shortwave radiation (a–d) for early summer,
mid–late summer, and early fall of 2015 (these periods are defined in Sect. 3.4).
High values on the x axis indicate completely overcast or cloudy
conditions, whereas as low values indicated clear skies. LRU increases with
increasing fdiff during each period, but the increase is most pronounced in the
early summer (e). Gc increases from clear to partly cloudy conditions across
the three periods and plateaus during overcast sky conditions (f). Vertical
bars indicate 1 SE. Across the three periods, LRU increased with
Gc and leveled off at Gc values greater than ∼ 0.5 mol m−2 s−1(g).

Figure 7Daytime means (defined as periods when PAR >100µmol m−2 s−1) for three heat wave periods (plotted as red,
yellow, and purple, while the overall time series is shown in blue).
Variables displayed are canopy temperature (∘C; a), VPD-leaf (b),
FOCS(c), NEE (d), water vapor flux (e), and canopy conductance (Gc,
f). Units for each panel are the same as specified in previous figures.

3.4 Sensitivity to diffuse light

Midday fluxes of OCS and CO2 were
found to be sensitive to changes in the fraction of diffuse : total incoming
shortwave radiation (fdiff; Fig. 6b–c). For these analyses, data were separated
into three periods corresponding to early summer (day of year, DOY 109–180), mid–late
summer (DOY 180–240), and early fall (DOY 240–297) and were binned into three
categories: clear-sky conditions, partly cloudy, and overcast (defined in
Sect. 2.7). Midday VPD was highest under clear-sky conditions and lowest
under overcast skies but was most different across the three periods during clear skies (Fig. 6a). Consequently, OCS and CO2 uptake was
highest (most negative fluxes) under overcast conditions during the early
summer, and generally declined as fdiff decreased across all time periods
(Fig. 6b–d). Across the three periods, the rate of decrease was much higher as
fdiff changed from partially cloudy to clear. During the mid–late summer, however (red diamonds in Fig. 6a–f), the diffuse light effect resulted in GPP and
NEE being almost as high as during the early summer. FOCS was also
highest under partially cloudy skies during this time and only showed a
very weak decline under completely overcast conditions. Overall, the
behavior of OCS and CO2 fluxes was similar during the later time
periods. LRU (calculated according to Eq. 5) was
lowest under partly clear skies and highest under overcast conditions. This
is because under highly diffuse conditions, carbon uptake is additionally
limited by light, whereas FOCS is not (Wehr et al., 2017; Maseyk et
al., 2014). The shape of the LRU curves can additionally be explained by
examining canopy conductance (Gc; Fig. 6f), which was also higher under
overcast skies. LRU increased with Gc across all three periods (Fig. 6g) and appeared to be constant for Gc greater than ∼ 400 mmol m−2 s−1.

The diffuse light enhancement of stomatal and canopy conductance is well
documented across a range of forest ecosystems
(Alton et al., 2007; Cheng et al., 2015; Hollinger et al., 2017; Urban et al.,
2007; Wharton et al., 2012). Lower VPD (Fig. 6a) and light levels allow
plants to keep stomata open at midday and continue fixing CO2. Lower
VPD reduces transpirational losses, and the lack of VPD-induced partial
stomatal closure reduces the resistance to CO2 diffusion into the leaf.
Correspondingly, the less directional nature of diffuse solar radiation
allows greater penetration into the canopy, thus increasing photosynthesis
across the entire canopy, even as a reduction in canopy top leaf
photosynthesis is observed due to a reduction in total radiation. In a
multiyear analysis at Wind River, Wharton et al. (2012) found that cloudy
and partly cloudy sky conditions during the peak-growing season lead to an
increase in CO2 uptake. During our study, Gc was generally higher in
the early growing season but increased as sky conditions changed from clear
skies to overcast. This increase was similar across the three time periods,
even as the response of OCS and CO2 fluxes was different across these
periods. This indicates that declining soil moisture (Fig. 3b–c) potentially
limits gas exchange as the summer progresses, even as canopy conductance can
be reasonably high under overcast skies. It is important to note that in the
absence of concurrent leaf and root water potential measurements, it is not
possible to attribute reduction in gas exchange to declining soil
moisture.

3.5 Response to heat waves

The year 2015
was the warmest over large parts of the Pacific Northwest since records began in the 1930s (Dalton et
al., 2017). We observed three distinct heat waves during the 2015 summer.
These were in early June (DOY 157–160), end of June–early July (DOY
175–188), and late July–early August (DOY 210–213). The three heat waves are
shown as red, yellow, and dark purple bars in Fig. 7; the overall time series
is shown in blue (daytime means are plotted for all variables, where daytime
is defined as PAR exceeding 100 µmol m−2 s−1). Additionally,
box plots for “non-heat-wave” and “heat wave” days are shown (labeled as “No
HW” and “HW”, respectively). Midday temperatures exceeded 30 ∘C during
these heat wave events, while VPD-leaf exceeded 3 kPa during the first heat
wave and increased to a mean daily maximum of 5.1 kPa during the last event
(Fig. 7b). The canopy was a net source of CO2 during all three events,
while midday means for NEE were usually negative (implying CO2 sink)
before and after the heat wave periods (Fig. 7c). During the first event,
FOCS was similar to days immediately preceding it (Fig. 7d). The third event
led to a reduction in FOCS, even though the canopy had received some
rainfall in the preceding weeks (Fig. 1c). Overall, mean daytime OCS uptake
decreased from −27 (pmol m−2 s−1) on non-heat-wave days (daytime
means presented as blue bars in Fig. 7) to −16 (pmol m−2 s−1) during heat wave days (daytime means from data presented as red, yellow, and
purple bars in Fig. 7). Water vapor fluxes (Fig. 7e) increased during the
first heat wave, compared to days immediately prior. The increased water
vapor flux is likely to form an increase in transpiration under high VPDl
(red bars in Fig. 7b), which ensures a steady transpirational flux (purple
bars in Fig. 7e). FH20 was not significantly different between
heat wave and non-heat-wave days (box plots in Fig. 7e) even as VPDl
was significantly higher during these events, leading to a suppression in
canopy conductance (Fig. 7f).

Over hourly, daily, and seasonal timescales, estimates of FOCS
generally tracked fluctuations in GPP, implying stomatal control of carbon,
water, and OCS fluxes at the site. We used continuous in situ measurements of
OCS mixing ratios, collocated measurements of water vapor fluxes and air and
canopy temperatures to calculate OCS uptake. We found the forest to be a
large sink for OCS, with sink strength peaking during daylight hours. The
mean LRU was ∼ 4 and varied in response to changing light conditions
and canopy conductance. These LRUs are larger than observed from other
ecosystem-scale studies but well within the range of reported values (Whelan
et al., 2018; Sandoval-Soto et al., 2005). The forest surface was found to be
a soil-moisture-dependent sink of OCS. Ecosystem fluxes of OCS and
CO2 were found to be strongly sensitive to the ratio of
diffuse : direct radiation reaching the top of the canopy. Uptake of both
OCS and CO2 increased as sky conditions changed from clear to partly
cloudy. A much smaller increase in uptake was observed as sky conditions
changed from partly cloudy to overcast, except during the early summer, when
soil moisture was not limiting. This change was mediated by the sensitivity
of stomata to changing cloudiness and soil moisture, as estimated from canopy
conductance. Finally, we examined the response of OCS, CO2, and
H2O fluxes on heat waves, and found that sequential heat waves lead
to suppression in the stomatal gas exchange of OCS and CO2 fluxes, but
not in the flux of water vapor.

Our results support the growing body of work that suggests ecosystem-scale
OCS uptake is controlled by stomatal dynamics. While moist old-growth
forests in the Pacific Northwest of the US do not represent a very large fraction
of the global terrestrial surface area, results from this study are likely
relevant for other old-growth forests, particularly high-LAI and very wet
forests with extensive epiphyte cover, which are widespread in the humid
tropics.

Funding was obtained by CS. Data collection was carried out by BR, MB, DN, CS, and MW. The methodology was developed by BR, CS,
FM, and MB. Formal analyses were carried out BR, SW, and MW. All authors contributed to writing, reviewing and editing the paper.

This work was partly funded by NASA SBIR
Phase II award NNX12CD21P to LGR, Inc. (“Ultrasensitive
Analyzer for Realtime, In-Situ Airborne and Terrestrial Measurements of OCS, CO2, and CO.”), and an LGR instrument was used to collect the data reported in the study.

We would like to
thank the US Forest Service and the University of Washington for letting us
use the research facility at Wind River. In particular, we wish to sincerely
acknowledge Ken Bible and Matt Schroeder for their help with setting up the
experiment as well as maintenance throughout the measurement campaign.

Carbonyl sulfide (OCS) has gained prominence as an independent tracer for gross primary productivity, which is usually modelled by partitioning net CO2 fluxes. Here, we present a simple empirical model for estimating ecosystem-scale OCS fluxes for a temperate old-growth forest and find that OCS sink strength scales with independently estimated CO2 uptake and is sensitive to the the fraction of downwelling diffuse light. We also examine the response of OCS and CO2 fluxes to sequential heat waves.

Carbonyl sulfide (OCS) has gained prominence as an independent tracer for gross primary...